Such fiber couplers for geometric coupling of laser radiation conducted in the optical fibers are formed for example by drawing and merging the optical fibers. To achieve a high brilliance in the output beam bundle exiting the fiber coupler, the ratio of core diameter to overall diameter of the optical fibers can be increased up to the output side in the coupler, as described e.g. in EP 2 071 376 A1. Alternatively, the ratio of the mode field diameter of the laser radiation conducted in the optical fibers to the core diameter in the coupler can be increased (US 2010/0278486 A1, US 2010/0189138 A1) wherein, during a tapering by drawing the optical fibers by more than the factor of three, complex refractive index structures are required to achieve a high brilliance.